MySQL and time zones

It turns out that MySQL has support for time zones, but doesn't have the necessary information to support named time zones out of the box. Once we have that sorted, there are some new and exciting ways we can query local times while storing them as UTC in MySQL.

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It turns out that MySQL has support for time zones, but doesn’t have the necessary information to support named time zones out of the box. The instructions on getting time zone support are described in MySQL Server Time Zone Support section of the manual, but the short version is to run this script and then pass the results into MySQL to create the appropriate tables in the mysql db:

mysql_tzinfo_to_sql /usr/share/zoneinfo |mysql -u root mysql

This resets the contents of the time_zone, time_zone_name, time_zone_transition and time_zone_transition_type tables in the system mysql database. It constructs the tables from your operating system’s time zone information database, so if that ever changes, it would be prudent to re-run this script. (Note to self: I need a puppet manifest for updating it when the tzdata package is updated…!)

Having done this, I now get access to the symbolic names for time zones. Which is useful for using CONVERT_TZ:

SELECT CONVERT_TZ('2012-06-01 12:00:00', 'America/Los_Angeles', 'Europe/London');

which will give us back 8pm London time. Better still, the time zones are then DST aware. So, I can now do:

SELECT CONVERT_TZ('2012-06-01 12:00:00', 'Etc/UTC', 'Europe/London');

and get back 1pm local time.

There are a couple of situations where this can be useful. From here on, I’m assuming that you use DATETIME columns and that you normalise all these DATETIME columns to UTC because I use Rails and that’s what Rails ActiveRecord does. See the last paragraph where I discover TIMESTAMP and wonder if this is a sensible pattern in the first place.

The first situation is where you’re taking inputs in local time (local time in this example is Europe/London, which only gets interesting when DST is in effect in the summer). If you do:

SELECT * FROM readings
  WHERE started_at = '2012-06-01 12:00:00';

then you are implicitly assuming that the argument your passing is already in UTC. What if it’s in the user’s local time? Well:

SELECT * FROM readings
  WHERE started_at = CONVERT_TZ('2012-06-01 12:00:00', 'Europe/London', 'Etc/UTC');

which will convert the time to UTC before performing the query. I think that since we’re converting the input value and leaving the (indexed) column alone in the WHERE clause, this isn’t going to have any pejorative impact on index use. Of course, if ActiveRecord is your hammer, you’re already used to the process of converting to UTC time for SQL queries.

The second situation is in converting the results back to local time. If you do:

SELECT started_at FROM readings;

then you’re going to get back the readings in UTC. However, if you do:

SELECT CONVERT_TZ(started_at, 'Etc/UTC', 'Europe/London') AS local_started_at
  FROM readings;

then you get the time back as a local time. If ActiveRecord is your thing, this is all handled inside it, so you don’t need to worry about it at the DB level.

The third situation is a little more interesting. Say you’re doing date based aggregation in SQL (grouping by YEARWEEK or WEEKDAY for example). If you do something along the lines of:

SELECT YEARWEEK(started_at) AS year_week_started_at, SUM(value) AS value
  FROM readings
  GROUP BY YEARWEEK(started_at);

then your results will be subtly wrong because you’re grouping into weeks based on the UTC time. In the case of BST, the hour at the start and end of the week is being allocated to the wrong week… So, instead we convert:

SELECT YEARWEEK(CONVERT_TZ(started_at, 'Etc/UTC', 'Europe/London')) AS year_week_started_at, SUM(value) AS value
  FROM readings
  GROUP BY YEARWEEK(CONVERT_TZ(started_at, 'Etc/UTC', 'Europe/London'));

Now we’re correctly grouping by local time.

From an application perspective, I would imagine setting up these queries to pass in the “local” time zone based on the user’s time zone setting – perhaps grabbed from the user’s model, or the session.

Of course, MySQL has a built in type called TIMESTAMP which does most of this work for us. It stores timestamps internally as UTC, but converts them on the fly into the session time_zone setting. I’m not sure if that safely covers the grouping behaviour in the third example. Sadly, being an ActiveRecord weenie, I think I’m stuck with the convention of using DATETIME columns and the UTC/local time conversion happening inside ActiveRecord. Which means that when I need to dig down for performant SQL queries with times, I need to be aware of time zone issues.

Things I still need to figure out (help?):

  • Whether somebody has already tweaked ActiveRecord to make use of time zone aware TIMESTAMP columns natively instead of DATETIME columns with internal conversion. Or, if not, how hard it would be to implement. I’d imagine having something that wraps the request and sets the time zone on the AR session(s) to the user’s preferred time zone, then carries on its queries as usual, but gets back a timestamp which includes the time zone information and creates a TimeWithZone instance from that…?

  • If there is any pejorative impact on index use with any of the changes to these queries. Or any other horrible performance implications.

  • If using TIMESTAMP columns solves the grouping issue covered in the third situation.

Chances are I’ll be ‘refactoring’ an application based on this research in the next couple of days, so there may be an update or two to this post as that happens. :)

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